2D/2D Bi2Se3/SnSe2 heterostructure with rapid NO2 gas detection

Heterostructure engineering is crucial for enhancing gas sensing performance. However, achieving rapid response for room-temperature NO2 sensing through rational heterostructure design remains a challenge. In this study, a Bi2Se3/SnSe2 2D/2D heterostructure was synthesized by hydrothermal method for the rapid detection of NO2 at room temperature. By combining Bi2Se3 nanosheets with SnSe2 nanosheets, the Bi2Se3/SnSe2 sensor demonstrated and the lowest detection limit for NO2 a short response time (15 s) to 10 ppm NO2 at room temperature, reaches 25 ppb. Furthermore the sensor demonstrates significantly larger response to NO2 than to other interfering gases, including 10 ppm NO2, H2S, NH3, CH4, CO, and SO2,demonstrating its outstanding selectivity. And we discuss the mechanism of related performance enhancement.


Introduction
Flexible sensors for pressure, temperature, humidity, strain, and heat flux have garnered to further enhance the sensing performance of gas sensors based on SnSe 2 .For instance, Li et al. fabricated a chemiresistive sensor based on a SnSe 2 /SnO 2 heterojunction using the thermal oxidation method for the detection of NO 2 gas (Li et al., 2020).While significant advancements have been achieved in enhancing the sensing capabilities of SnSe 2 for NO 2 detection, there is still a need for further improvement, particularly in terms of reducing the response time, as well as improving selectivity.An important limitation of two-dimensional materials for room temperature gas sensing is the long response recovery time due to the slow desorption process.It is well understood that in comparison to 0D/2D and 1D/2D heterostructures, 2D/2D heterostructures offer closer interfacial contacts, facilitating enhanced charge transfer and thereby improving sensing properties.It is anticipated that the development of these 2D/2D heterostructures on SnSe 2 could efficiently accelerate the response time.
Enhancing selectivity is a critical factor for gas sensing equipment.Current strategies employed to improve the selectivity of gas sensing devices encompass precious metal modification, heterostructural construction, ultraviolet irradiation, and external auxiliary heatin (Majhi et al., 2015;Zhang et al., 2016;Joshi et al., 2018) Bi 2 Se 3 , a representative 2D semiconductor, has gained significant interest across various fields like optoelectronic circuit, photocatalysis, and batteries due to its favorable bandgap, exceptional stability, and ease of synthesis (Huang et al., 2020;Li et al., 2022).Moreover, the lower work function of Bi 2 Se 3 compared to SnSe 2 promotes facilitates electron migration from Bi 2 Se 3 to SnSe 2 , resulting in a decrease in the work function of SnSe 2 .This reduced work function of SnSe 2 facilitates electron transfer to NO 2 molecules during sensing (Li et al., 2024).Additionally, the remarkable catalytic properties of Bi 2 Se 3 contribute to lowering the activation energy needed for gas sensing, ultimately enhancing sensing performance, particularly in reducing response time and increasing selectivity.Consequently, the engineering of 2D/2D heterostructures by combining Bi 2 Se 3 nanosheets and SnSe 2 holds promise for developing sensors with improved sensitivity and rapid response at room temperatures.
In this study, 2D/2D heterostructures of SnSe 2 /Bi 2 Se 3 were synthesized using a combination of colloidal method and solvothermal method with a laminated stack structure.The optimized SnSe 2 /Bi 2 Se 3 sensor demonstrated a notable reduction in response time for 10 ppm NO 2 , dropping from 73 to 15 s, and a decrease in recovery time from 300 to 110 s.The optimized sensor also exhibits a high response to NO 2 , with a high response rate of 130% to 10ppm of NO 2 .These improved sensing capabilities can be ascribed to enhanced transfer of charges and the larger number of adsorption sites, which are both enabled by the SnSe 2 /Bi 2 Se 3 2D/2D heterostructures.Additionally, the fatigue tests of the sensor after 100, 500, and 1,000 cycles of bending and relaxation showed no significant decrease in response values, demonstrating its excellent mechanical performance.These findings present a novel and effective strategy for developing a practical NO 2 gas sensor based on SnSe 2 .

Synthesis of SnSe 2 nanoplates
Firstly, 0.5 mmol SnCl 4 •5H 2 O, 1.5 mmol SeO 2 , and 0.5 mmol 1, 10 -phenanthroline were dissolved in a three-neck flask.The mixture was stirred for 10 min under a highly pure N 2 atmosphere at room temperature.Following a 10-min N 2 degassing, the solution underwent heating to 110 °C while being stirred magnetically for an additional 10 min.Then, further increased to 260 °C and maintained for another 30 min with continuous stirring.Cooling down to room temperature, the resulting products were collected and centrifuged under three rounds of washing: first with cyclohexane and then with ethanol.Ultimately, the products were kept at 70 °C for 10 h.

Synthesis of Bi 2 Se 3 /SnSe 2 heterostructures
The preparation of Bi 2 Se 3 /SnSe 2 heterostructures involved a simple solvothermal method.Its synthesis process is shown in Figure 1A.Precisely, 0.2 mmol of the pre-synthesized SnSe 2 was gently dispersed in 20 mL ethanol and stirred for 0.5 h.Subsequently, a specific quantity of Bi(NO 3 ) 3 •5H 2 O was added to the suspension and vigorously stirred for another 0.5 h.The resulting mixture was then moved to a 25 mL Teflon-lined autoclave and the solvothermal reaction was performed at 180 °C for 12 h.Finally, the resulting product was collected and subjected to three washes with ethanol and water.
For simplicity, the Bi 2 Se 3 /SnSe 2 heterostructures were labeled as BS-1, BS-2, and BS-5, reflecting the amounts of Bi salt added (0.020, 0.025, and 0.040 mmol) respectively.In contrast, the synthesis of pure Bi 2 Se 3 involved the addition of 1 mmol Bi 2 O 3 , 3 mmol SeO 2 , and 0.4 g NaOH into 40 mL of ethylene glycol.After being vigorously stirred for 1 h, the resulting suspension was placed in a Teflon-lined autoclave and the reaction was performed at 180 °C for 7 h.Subsequently, the resultant products were collected, subjected to multiple washes with ethanol and deionized water, and finally dried at 60 °C for 10 h.

Characterization of materials
The morphologies of the samples were inspected using both FESEM (SIRION 200) and TEM (JEOL-1400).Elemental composition was characterized by EDX and elemental mapping, while crystal structure was analyzed by XRD measurements (Bruker D8 Advance).The chemical states were determined through XPS analysis using an ESCALAB 250 spectrometer.

Gas-sensing properties
The gas sensors were produced through applying a solution of sensing materials through drop-casting (10 mg/mL in ethanol) onto Ag-Pd interdigital electrodes on an alumina substrate measuring 6.6 × 6.0 mm.To evaluate the gas sensing performance, a homemade sensor-testing system was employed, and a simplified experimental setup is depicted in Supplementary Figure S1.The real-time changes in conductivity of the sensors was collected via the electrochemical workstation (CHI 630E).In the sensing experiments, a precise volume of test gas was injected into the 4 L test chamber via a syringe.The relative humidity levels were regulated using a CK-80G commercial humidity chamber by Kingjo.All tests were performed in room air at 25 °C with a relative humidity of 40%-50%.The sensing response (S) was determined using the equation of S = (R g -R a )/R a , where R g and R a represented the sensor resistance in the target gas and in air, respectively.The response and recovery times represent the duration it takes for the sensor to reach 90% of the resistance change after a target gas was injected and released, respectively.

Manufacturing of flexible chemical sensor
The flexible chemical sensor, incorporating Bi 2 Se 3 /SnSe 2 heterostructures, was intricately applied to a polyethylene terephthalate substrate with interdigital gold patterns.The fabrication procedure for the device is meticulous and mirrors a method similar to the one previously documented.(Wang et al., 2016;Wang et al., 2017).

Material structure and morphology
Characterized by SEM, TEM, and HRTEM.Figures 1B, E display the SEM images of SnSe 2 and BS-2 samples, respectively, and their insets are the corresponding TEM images.The SnSe 2 exhibits nanoplates morphology with the uniform dimension of Response curves of the sensors based on SnSe 2 , Bi 2 Se 3 and Bi 2 Se 3 /SnSe 2 heterostructures to 10 ppm NO 2 at room-temperature.500-800 nm.After the solvothermal process, the SnSe 2 still retain its original shape, and the hexagonal Bi 2 Se 3 nanoplates grow on the surface of SnSe 2 , forming a heterostructure with intimate contact.The HRTEM images in Figures 1C, F clearly demonstrate the presence of a heterojunction interface.The lattice spacings of 0.310 and 0.225 nm belong to the (002) and (012) planes of SnSe 2 , respectively, while the 0.360 nm spacing corresponds to the (101) plane of Bi 2 Se 3 .In addition, the EDX mapping results demonstrate the uniform distribution of Sn, Se, and Bi elements (Figures 1D, G), which further indicates the formation of interconnected structures between SnSe 2 and Bi 2 Se 3 .
Frontiers in Chemistry frontiersin.org Se, and Bi elements (Figure 2B), which is in line with the findings from the EDX mapping.In the detailed XPS spectrum of Sn3d, peaks at 485.9 and 494.3 eV are corresponding to Sn 3d 5/2 and Sn 3d 3/2 (Figure 2C), respectively, indicating the presence of Sn 4+ in SnSe 2 .Two peaks around 52.8 and 53.6 eV belong to the 3d 5/2 and 3d 3/2 of Se 2-species (Figure 2D), respectively.In the Bi 4f and Se 3p spectra presented in Figure 2E, the distinctive peaks at 157.8 and 162.8 eV are indicative of Bi 4f 7/2 and Bi 4f 5/2 , respectively, indicating the chemical state of Bi 3+ .The peaks that located at 159.7 and 165.4 eV could be separately assigned to the Se 3p 3/2 and Se 3p 1/2 orbitals of Se 2- (Xu et al., 2019).Notably, in comparison with pristine SnSe 2 , the binding energy of Sn 3d in BS-2 slightly shifts to higher energy, while the binding energy of Bi 4f in BS-2 are slightly lower than that of pure Bi 2 Se 3 .Such migration shifts of the binding energies may be ascribed to the change in electron density on the surfaces of the samples, and the above results confirm that the electrons in the heterostructure transfer from SnSe 2 to Bi 2 Se 3 .The analyses conducted affirm the successful creation of Bi 2 Se 3 /SnSe 2 heterostructures.

Gas sensing properties
The NO 2 sensing capabilities of Bi 2 Se 3 /SnSe 2 heterostructures were examined through testing the change in resistance upon exposed to target gases.The initial investigation focused on assessing the influence of the Bi 2 Se 3 to SnSe 2 ratio on the sensing performance of the heterostructures.As the ratio of Bi 2 Se 3 increased, the sensing response increases first then decreased (Figure 3).The BS-2 performs the most significant sensing response to NO 2 .By contrast, the sensors based on pure SnSe 2 and Bi 2 Se 3 exhibit inadequate response and sluggish recovery.The significantly improved sensing performance may be credited to the formed heterointerface between SnSe 2 and Bi 2 Se 3 , which facilitates charge transfer and provides abundant active sites.According to the detailed sensing characteristics outlined in Supplementary Table S1, the Bi 2 Se 3 /SnSe 2 -2 sensor has been chosen for further research due to its impressive performance, showing the largest response rate of 130% and the shortest response/recovery times of 15/110 s to 10 ppm NO 2 .
The dynamic response curve of BS-2 sensor toward different NO 2 concentration was further measured.As illustrated in Figure 4A, the sensor shows excellent response and recover ability, and the lowest detection limit for NO 2 reaches 25 ppb.Besides, the correlation equation relating response value to gas concentration at ppb (Figure 4B) and ppm level (Figure 4C) could be separately calculated as a good linear relationship, which suggests its promising potential for sensor calibration purpose.Additionally, the wide detection range of the BS-2 sensor enables it to have a broad range of applications.
The selectivity of the BS-2 sensor was studied upon exposure to different analytes, including 10 ppm NO 2 , H 2 S, NH 3 , CH 4 , CO, and SO 2 .As shown in Figure 5A, the sensor demonstrates significantly larger response to NO 2 than to other interfering gases, demonstrating its outstanding selectivity.To assess long-term stability, the responses of the BS-2 sensor to 10 ppm NO 2 were recorded at a 5-day interval.The sensor exhibits a slight response recession (Figure 5B) and nearly identical sensing behaviors toward target gases within 50 days (Supplementary Figure S3), thereby affirming its significant stability and reliability.In addition, the BS-2 sensor presents notable stability when working in moderate humidity levels (25%-55%), making it suitable for practical applications (Figure 5C).
For a comprehensive assessment of sensing ability, Table 1 sums up a comparison between the optimal sensor in this study and other reported sensors based on 2D materials.The Bi 2 Se 3 /SnSe 2 sensor shows superior NO 2 sensing performance, featuring minimal power usage and heightened sensitivity, and notable response/recovery characteristics.
The sensor based on Bi 2 Se 3 /SnSe 2 demonstrates outstanding NO 2 sensing performance, showcasing minimal power usage and heightened sensitivity and remarkable response/recovery characteristics.To explore the potential capability, the mechanical flexibility properties of the BS-2 sensor were further studied.The flexible gas sensor was produced by depositing the Bi 2 Se 3 /SnSe 2 heterostructures onto a PET substrate with gold interdigitated electrodes (Figures 6A-C).The gas sensing characteristics of the sensor under bending angle of 30 °were then determined.As displays in Figure 6D the dynamic response and recovery curves in each trial are consistent with the flat condition.Furthermore, the fatigue test of the sensor after 100, 500, and 1,000 cycles bending and relaxing processes shows no significant degradation in response values (Figure 6E).The results presented above showcase the promising application potential of the flexible Bi 2 Se 3 /SnSe 2 sensor in wearable sensing devices.

Gas sensing mechanism
The gas sensing mechanism of semiconductive materials is established on the modification of resistance caused by the interaction between gas molecules and the sensing materials.(Zappa et al., 2018;Bag and Lee, 2019).The sensing mechanism of SnSe 2 has been extensively discussed in the literature.As depicted in Figure 7, when exposed to air, O 2 molecules adhere to the surface of n-type SnSe 2 , resulting in the creation of O 2− (ads) by capturing free electrons from the conduction band of SnSe 2 .At temperatures below 100 °C, the main form of adsorbed oxygen is O 2− .The redox reaction described above is illustrated in Eq. 1. (Kim et al., 2017).
When SnSe 2 is exposed to NO 2 , the gas molecules can efficiently capture electrons from the conduction band of SnSe 2 owing to the greater electronegativity of NO 2 in comparison to O 2 .Furthermore, the reaction process can be delineated by Eqs 2, 3 below.Nevertheless, unmodified SnSe 2 demonstrates a diminished response when compared to Bi 2 Se 3 /SnSe 2 .This phenomenon arises from the relatively low coverage of chemisorbed oxygen on the pristine SnSe 2 surface.Consequently, NO 2 can directly withdraw electrons from SnSe 2 .(Li et al., 2020).As a result, physical adsorption (Eq. 3) predominantly influences the sensing process. NO The superior NO 2 sensing properties of Bi 2 Se 3 /SnSe 2 heterostructures are primarily attributed to the following factors building upon the foundational sensing mechanism discussed earlier: Initially, the created n-n heterojunction between Bi 2 Se 3 and SnSe 2 plays a vital part in boosting the sensing response of the heterostructures.The electronic effects resulting from the construction of the SnSe 2 /Bi 2 Se 3 heterojunction heterostructure are primarily attributed to band alignment.As depicted in Figure 8, the work function of Bi 2 Se 3 is 4.3 eV, while the work function of SnSe 2 is 4.9 eV.Therefore, the Fermi level of Bi 2 Se 3 is notably higher than that of SnSe 2 .Consequently, upon the formation of the heterojunction, electrons transfer from Bi 2 Se 3 to SnSe 2 , and holes transfer from SnSe 2 to Bi 2 Se 3 , leading to the separation of holes and electrons.This results in the generation of an electron accumulation layer on the SnSe 2 side and an electron depletion layer on the Bi 2 Se 3 side, leading to electron accumulation on one side of SnSe 2 and an improvement in the electronic structure at the interface.
During the sensing process of NO 2 gas-sensitive materials, when NO 2 gas molecules come into contact with the material surface in a detection gas environment, the gas is first adsorbed by the material.Due to the oxidizing nature of NO 2 , it then captures electrons from the surface of material, resulting in a change in the carrier concentration and leading to variations in resistance and circuit current.The sensing process can be divided into two primary steps: gas adsorption, where active sites on the material surface adsorb gas, and electron reaction, where gas interacts with electrons on the material surface.(Gong et al., 2019).
In the 2D-2D SnSe 2 /Bi 2 Se 3 heterostructure material, where the sensing material remains SnSe 2 , the aggregation of electrons on the surface of SnSe 2 after heterostructure equilibrium is achieved enhances the interaction between NO 2 gas and the material.This leads to a larger change in resistance signal, thereby improving the sensitivity of the material to NO 2 gas.The synthesized Bi 2 Se 3 /SnSe 2 2D/2D heterostructure is lamellae-stacked, which has relatively more adsorption sites and large specific surface area, which is conducive to the adsorption and desorption of NO 2 .Moreover, the heterostructure exhibits significantly reduced response and recovery time duing to the enhanced electron transfer rate within the material.

Conclusion
In summary, Bi 2 Se 3 /SnSe 2 2D/2D heterostructures are successfully synthesized and used for NO 2 detection.The optimized Bi 2 Se 3 /SnSe 2 heterostructure exhibited rapid response towards NO 2 gas.Compared to pure SnSe 2 , the response time was significantly reduced from 73 to 15 s (10 ppm).The enhanced sensing performance is a direct result of the abundant n-n heterojunctions, improved interface charge transfer, and increased active sites that are inherent in the SnSe 2 /Bi 2 Se 3 heterostructure.Furthermore, the sensor displayed excellent selectivity, with a low detection limit of 10 ppb and a broad detection range from 10 ppb to 20 ppm.Furthermore, the fatigue test of the sensor after 100, 500, and 1,000 cycles bending and relaxing processes shows no significant degradation in response values.These results offer important insights for selecting materials and designing heterostructures to achieve effective room temperature gas detection in a range of applications.
FIGURE 3 FIGURE 4 (A) Dynamic response curves of BS-2 sensor to NO 2 (20 ppm-25 ppb); (B, C) The correlation between gas concentration and sensor response value.
FIGURE 5 (A) Selective responses of BS-2 sensor to 10 ppm NO 2 , H 2 S, NH 3 , CH 4 , CO and SO 2 ; (B) Long-term stability and (C) effects of humidity on the sensing responses of BS-2 sensor to 10 ppm NO 2 .
), and (205) crystallographic planes of Bi 2 Se 3 .With respect to the Bi 2 Se 3 /SnSe 2 heterostructures, an increase in the amount of added Bi salt led to an augmentation in the diffraction peaks of Bi 2 Se 3 and a simultaneous decrease in the peak assigned to SnSe 2 , without any noticeable impurities, thus indicating the high purity of these synthesized products.XPS measurement was performed to analyze the chemical compositions and chemical states of the samples.The survey spectra of the BS-2 sample demonstrates the co-existence of Sn,

TABLE 1
Comparison of NO 2 responses of Bi 2 Se 3 /SnSe 2 sensor with that of other materials reported in literature.